The invention relates to a method of manufacturing a thin layer detector for an integrating solid state dosimeter and a detector manufactured according to this procedure.
In integrating solid state dosimeters, electrons which have been liberated during an ionizing irradiation are captured and stored in atomic trapping sites of the detector material. Their release can be stimulated, e.g., by heating the detector in the read-out process, and is accompanied by the emission of photons, called thermoluminescence (TL), and sometimes by the emission of electrons, called thermally stimulated exoelectron emission (TSEE). The quantity of emitted particles provides a measure of the radiation dose received previously. Besides some sulfates and oxides, certain inorganic salts, such as lithiumfluoride (LiF) doped with activators like magnesium (Mg) and titanium (Ti), are applied to devices for thermoluminescence dosimetry (TLD) and form a main subject of this invention. Generally, the detector fabrication starts from powders of these substances.
Sometimes, the TL-powder is contained loosely in a capsule during radiation exposure and is redistributed onto a heating tray during read-out. In general, however, the powders are processed into a solid detector.
According to a further known process, the handling of TL-powder is facilitated by embedding the powder permanently in a plastic matrix such as polytetrafluoroethylene, (PTFE), as sold for example under the trademark Teflon. Since the TL-powder in such an arrangement constitutes only up to 30% of the detector volume, the radiation response is considerably lowered. Further, since the PTFE disk may curl off the heating tray, special care must be taken with regard to the thermal contact. Moreover, to avoid detector deterioration, the temperature must not exceed 300.degree. C., whereas post-irradiation annealing should be conducted at up to 400.degree. C. for regeneration of the detector. Other problems of PTFE-based detectors are related to conserving optical transparency and avoiding electrostatic dust attraction.
More frequently, solid detectors are fabricated by compression of the powder at elevated temperatures in a vessel, and then extruding the fused material through a die. Chips of appropriate shape are sliced off the extrusion and polished, thus obtaining compact, self-supporting detectors. However, owing to the physical fragility of such detectors their thickness must not be lower than 0.5 mm or even 1 mm.
In a further known method of dosimeter manufacturing, the dry TL-powder is sprinkled onto a self-adhesive, heat resistant plastic tape (Kapton). However, the sensitivity and the stability of the radiation response of such detectors turned out to be rather low.
By suspending the detector powder in a liquid in a known manner, smooth sedimentation layers may be formed on an arbitrary substrate. The sedimentation may be forced by filtration, i.e. by drawing the liquid through the substrate which must be porous throughout in order to act as a filter body. To improve the adherence of the filtrated powder, a subsequent isostatic compression at room temperature is advised. In order to avoid high temperatures which may deteriorate the materials, this method has been developed particularly for the preparation of TSEE-detectors. A subsequent annealing is performed only if additional sintering of the powder grains in the ready layer is intended.
A known specific hot preparation method uses powder spraying through a chemical flame, so as to bond the particles with each other and with a suitable substrate. However, it turned out that flame temperature and composition are very critical parameters for the deposition of intact TL-detector layers.
From the above review of the existing diverse devices and processes, there still remains the problem of constructing a solid state dosimeter with a thin detector layer, which is not deteriorated with respect to its radiation response by a preparation procedure at elevated temperatures, e.g. up to 400.degree. C., and which is bonded permanently on a heat-resistant substrate, preferably of a low atomic number material.